Recently, display apparatuses using a field emission device (hereinafter, abbreviated as “FED”) have become promising candidates expected to be widely employed in household and industrial applications. FIG. 8 shows a cross sectional view depicting an exemplary Spindt type field emission unit 100 used as an electron emission source in a conventional FED, wherein the entire structure of the FED is not shown. The field emission unit 100 includes cathode electrodes 102 and gate electrodes 106 as essential electrodes. The cathode electrodes 102 and the gate electrodes 106 are formed by being deposited on a dielectric cathode substrate 101.
The cathode electrodes 101 made of a conductive material and cathode electrode wirings 103 are formed on and in contact with an upper surface of the cathode substrate 101. Further, a resistor layer 104 is formed on the cathode electrodes 102 and the cathode electrode wirings 103, and an insulating layer 105 is formed on and in contact with an upper surface of the resistor layer 104. Furthermore, the gate electrodes 106 made of a conductive material are formed on and in contact with an upper surface of the insulating layer 105. Above the cathode electrodes 102 are formed openings 107 in the insulting layer 105 and the gate electrodes 106, and emitters 108 of a trigonal pyramid shape are formed in the openings 107 to be in electrical contact with the resistor layer 104.
The cathode electrodes 102 are arranged in parallel in Y-direction (i.e., a direction toward a backside from a front side of the sheet of FIG. 8), and the gate electrodes 106 are arranged in parallel in X-direction (i.e., a direction from left to right in FIG. 8). Further, each of the cathode electrodes 102 is orthogonal to each of the gate electrodes 106, thereby forming a matrix.
An anode substrate (not shown) is installed to face an upper surface of the cathode substrate 101 at a specific distance on which the gate electrodes 106 are formed. Further, the anode substrate facing the field emission unit 100 includes an anode electrode (not shown) on which phosphor material is coated, and the anode electrode serves as a display plate. Further, the cathode substrate 101 and the anode electrode form a closed space, whose inside is maintained at a vacuum level.
Hereinafter, an exemplary operation of a FED having such configuration will be described. First, an electric potential that is positive with respect to the cathode electrodes 102 is applied to the anode electrode. Then, display data are assigned to a driver unit 112 (shown in FIG. 9) having first drivers respectively connected to the cathode electrodes. Meanwhile, an electric potential for making the emitters 108 emit electrons is applied to one of the gate electrodes 106 by using second drivers respectively connected to the gate electrodes 106 (not shown), and an electric potential is applied to the remaining gate electrodes 106 to prevent the emitters 108 thereof from emitting electrons.
Thus, electrons are emitted from gate emitters, i.e., a part of the emitters 108 installed in the openings 107 of the gate electrode 106 to which the electric potential for making the emitters 108 thereof emit electrons is applied, so that the electrons are ejected onto the anode electrode at positions corresponding to the respective gate emitters. Thus, the phosphor material in an area corresponding to the ejected positions emits light whose brightness depends on the display data, so that a single line display is performed in X-direction, i.e., in a direction in which the gate electrodes 106 are extended. In this manner, the gate electrodes 106 are scanned, i.e., sequentially selected one by one as a selected gate electrode to which a selection potential, i.e., an electric potential for making the emitters 108 thereof emit electrons, is applied and, at the same time, the display data corresponding to the scanned positions are assigned to the respective cathode electrodes 102, so that an image is displayed on an entire surface of the FED.
In such FED, the anode current is varied significantly depending on a change in the temperature thereof, thereby causing a change in the emission brightness.
FIG. 9 depicts a display apparatus capable of preventing an anode current from changing depending on a temperature change in a FED (see Japanese Patent Laid-open Application No. 2001-324955). With reference thereto, this display apparatus will be described in the following.
The display apparatus shown in FIG. 9 includes a FED 110; an anode current detector unit 111 for detecting an average current, i.e., an average value of an anode current flowing through an anode of the FED 110 over a specific period of time; a driver unit 112 for driving cathode electrodes that are functionally equivalent to the cathode electrodes 102 in FIG. 8; a display data output unit 113 for supplying a driving voltage to the driver unit 112 in accordance with display data; a display data amount detector unit 114 for counting an amount of the display data over a specific period of time; a reference voltage output unit 115 for generating and outputting a reference current, i.e., a reference value of the anode current based on the counted amount of the display data; a comparator 117 for comparing the average current with the reference current; a gate voltage control unit 118 for adjusting a voltage applied to gate electrodes that are functionally equivalent to the gate electrodes 106 in FIG. 8 if the average current is not same as the reference current; and a ROM 116 in which a table for generating the reference current is stored. Thus, the emission brightness is stabilized by adjusting the voltage applied to the gate electrodes 106 to control the anode current in response to the display data.
Herein, the anode current detector unit 111, the comparator 117 and the gate voltage control unit 118 form a feedback control system by which an output voltage of the gate voltage control unit 118 is automatically controlled in such a manner that an output voltage of the comparator 117 becomes 0, thereby restraining the temperature dependence of the emission brightness of the FED 110.
Since the above-described display apparatus stabilizes the emission brightness by using the feedback control system, the effect of such factors as a temperature change can be suppressed, so that its temperature characteristic is improved remarkably if the anode current is relatively large and the emission brightness of the FED 110 is relatively high.
However, if the emission brightness of the FED 110 is low, the detected current is very small, and therefore it becomes difficult to control the brightness. To be more specific, a signal to noise ratio (SNR) of the anode current to be compared by the comparator 117 is reduced, and a blind zone of the anode current range which has been introduced to stabilize the feedback control system becomes too wide to be neglected in comparison with the anode current, thereby making it difficult to detect the anode current accurately.
Further, if the above-described method of controlling the brightness by using the feed-back is employed by a display apparatus when the emission brightness is as low as described above, the stabilization of the emission brightness may be even interfered in some cases.